|Author:||Charette and Beaudette, 2018|
|Geological province:||Churchill Province|
|Geological subdivision:||Ungava Orogen / Narsajuaq Arc|
|Lithology:||Migmatitized felsic-intermediate gneiss|
Table des matières
The Sainte-Hélène Complex was introduced by Charette and Beaudette (2018) to distinguish orthopyroxene-free tonalitic to dioritic lithologies with gneissic and migmatitic textures of the Narsajuaq Arc. Previously, these lithologies were mostly included in unit pPNAR1 of the Older suite and, in a smaller proportion, in unit pPNAR2 of the Younger suite described by St-Onge and Lucas (1992) and St-Onge et al. (1992).
The Sainte-Hélène Complex groups foliated to gneissic rocks at the amphibolite facies located north of the Sugluk Suture, whose composition varies from tonalite to diorite. In addition to the compositional variations that mark gneissosity within this complex, many injections and bands conformable with foliation produce a banded appearance. Locally, isoclinal folds with an axis parallel to the foliation affect the gneissosity and felsic bands. Open folds of metric amplitude, folding the entire banded gneissic sequence, were also observed, particularly near the coast southwest of Ivujivik.
The two main lithologies in this group were divided according to the dominant composition observed in outcrop, namely, migmatitized quartz diorite (pPshn1) and migmatitized greyish tonalite (pPshn2). These two units are commonly observed in transitional contact or as horizons in each other. The third unit (pPshn3) of Sainte-Hélène Complex has a characteristic gneissic texture. Diatexite (pPshn4), on the other hand, is mainly associated with units pPshn1 and pPshn2 and appears to be derived from their melting.
Quartz diorite is dark grey, fine to medium grained and granoblastic. Its appearance varies from gneissic to homogeneous. The gneissic texture, mostly observed in the Hudson Bay coast area, is produced by a transitional variation in the proportion of mafic minerals (15-25%). In areas where unit pPshn1 is more homogeneous, quartz diorite has little compositional variation, but commonly includes diffuse and lenticular leucosomes as well as small (0.5-1 cm) plagioclase crystals. In both the gneissic and homogeneous areas, lithologies of unitpPshn1 have characteristic banding. The latter is marked by two types of bands whose grain size contrasts with quartz diorite. First, leucocratic bands make up between 15 and 20% of lithology and are interpreted as leucosomes. These granitic to tonalitic bands, millimetric to centimetric, have the same grain size or are slightly coarser grained than the quartz diorite, with which they are in sharp contact. These felsic bands are locally folded or discontinuous and are generally conformable with gneissosity. In places, these bands become coarser and cut the planar fabric. It is possible to observe in places that these conformable and cross-cutting bands produce a typical metatexite mesh texture. Clusters of clinopyroxene and hornblende are observed within the coarser bands and, locally, orthopyroxene occurs as relic in the core of this hornblende. Second, dioritic bands or horizons richer in mafic minerals are also contained in quartz diorite. These are in sharp contact with the quartz diorite, making up to 5-15% of the outcrops and are, in places, boudinaged. They are centimetric to metric and are conformable with foliation.
In thin section, the matrix is moderate to strongly recrystallized and mafic minerals mark foliation. Hornblende is the main mafic mineral, followed by biotite, which, in a few thin sections, has a reddish colour. Sphene, clinopyroxene, apatite and opaque minerals, when observed, are present in significant proportions, while zircon and chlorite are present as trace minerals. When present, clinopyroxene is replaced by hornblende. Biotite, in addition to occuring as flakes, replaces hornblende. Chlorite is observed in some horizons and is associated with biotite flakes.
In the northern portion of the arc, quartz diorite is also composed of several bands, but has more intense deformation. It is mylonitized and has plagioclase porphyroclasts as well as quartz bands and, in places, millimetric hornblende clusters. In thin section, biotite, hornblende and clinopyroxene make up most of the mafic minerals. A second foliation is marked by biotite at approximately 25 to 30º from the main foliation.
Tonalite of the Sainte-Hélène Complex (pPshn2) is similar to unit pPshn1 because of its banded texture and the many types of bands that compose it. However, it is felsic and has homogeneous metric zones without bands. When homogeneous, tonalite is grey, fine grained and contains between 10 and 15% mafic minerals disseminated in the matrix. When banded, tonalite exhibits compositional variations and contains whitish to pinkish bands. The compositional changes are marked by diffuse decimetric horizons of tonalite or quartz diorite with higher contents of mafic minerals (25%) as well as centimetric to decimetric dioritic horizons in sharp contact, locally boudinaged, which account for less than 5% of outcrops. Bands, possibly of parallelled leucosomes, are granitic to tonalitic and millimetre to centimetre thick. They account for 10 to 30% of the lithology, are usually discontinuous and are observed in sharp to diffuse contact within tonalite. On some outcrops, these bands are folded or boudinaged. Very locally, coarse-grained mafic minerals are contained in these mobilisates. In addition to the different bands, melanocratic dioritic enclaves, centimetre to decimetre thick, are in sharp contact in tonalite. Locally, a mylonitic texture and K-feldspar porphyroclasts are observed over decimetric thicknesses.
In thin section, tonalite is usually well recrystallized. Mafic minerals are disseminated throughout the matrix and form continuous or discontinuous millimetric laminae in places. Typical mafic minerals are biotite and hornblende. Within the same thin section, some bands are richer in biotite and others in hornblende. As for plagioclase crystals, they are locally saussuritized. Muscovite is also observed in larger flakes in the matrix. Accessory minerals are zircon, apatite, allanite and sphene.
Granitic and granodioritic gneiss contain many pinkish granitic bands locally porphyroclastic. These bands account for 10 to 20% of outcrops and can sometimes reach up to 30-35%. Gneiss of the Sainte-Hélène Complex are light grey and pink grey in the most banded areas. They contain horizons of millimetric to centimetric K-feldspar porphyroclasts accounting for generally less than 10% of the lithology. The mylonitic fabric highlights an intense deformation that is characterized by tight isoclinal folds with an axial plane parallel to the fabric. The latter is associated with strong flattening and stretching (L/S tectonite). Migmatitized quartz diorite horizons have been described in some outcrops.
At the sample scale, the rock is generally granodioritic, but also has compositional variations that are marked by diffuse bands of granitic composition. In highly banded areas, the unit contains syenogranitic bands in sharp contact and conformable, as well as millimetric to centimetric K-feldspar porphyroclasts, giving locally an augen appearance. In more homogeneous areas, millimetric plagioclase porphyroclasts are evenly distributed in the matrix. In thin section, deformation results in an oriented granoblastic appearance and quartz bands with subgrain extinction. Mafic minerals, on the other hand, are concentrated in thin, discontinuous laminae. In some places, a reduction in grain size is noted in association with the latter. Major mafic minerals include biotite and hornblende in varying proportions. Generally unit pPshn3 contains many secondary minerals in association with the mafic laminae, such as sphene, epidote, opaque minerals and apatite.
Diatexite of the Sainte-Hélène Complex is heterogeneous and contains biotite schlierens on the edge of felsic bands. These represent approximately 10% of the outcrop. They are centimetre thick and granitic. The heterogeneity of the Saint-Hélène Complex diatexite is characterized by many folded leucosomes with a flow texture. Between whitish to pinkish leucocratic ribbons, diatexite contains numerous schlierens of mafic minerals. This unit is commonly characterized by variation in mafic minerals and grain size, which contributes to its heterogeneous appearance. Also, centimetric to decimetric enclaves of quartz diorite and diorite are partially to highly assimilated within migmatite. The diatexite protolith is difficult to identify, but since this unit forms zones in tonalite of the Sainte-Hélène Complex, it is interpreted as derived from partial melting of the latter.
Tonalite of the Sainte-Hélène Complex (pPshn2) is mainly distributed in and north of the Sugluk Suture, in the western portion of the Narsajuaq Arc, where it forms long and folded kilometric masses. It also forms a dome near Nallujaq Lake. In the northern portion of the arc, it forms smaller, one-kilometre-thick klippes. Quartz diorite (pPshn1) is associated with tonalite and a large elongated mass near the Sugluk Suture. Diatexite (pPshn4), on the other hand, forms a mass within unitpPshn2 north of the Sugluk Suture. At the far north end of the mapped area, the Sainte-Hélène Complex mainly occurs as mylonitized quartz diorite (pPshn1) klippes. These klippes form kilometric open folds and are associated with granitic to granodioritic gneiss (pPshn3).
Pending (Charette and Beaudette, 2018).
When occurring as klippes dispersed in the northern portion of the Narsajuaq Arc, units of the Sainte-Hélène Complex are in enclaves in tonalite of the Tasialuk Allipaaq Complex (pPali1) and in granitoids of the Sanningajualuk Suite (pPsnn1). They are in contact with gabbronorite and intermediate gneiss of uncertain origin of the Pingalasuit Complex (pPpgs1 and pPpgs2a) and metasedimentary rocks of the Erik Cove Complex (pPecv). Moreover, these metasedimentary rocks alternate locally with tonalite horizons of unitpPshn2. Tonalite and quartz diorite are also in contact with opdalite and enderbite of the Navaataaq Suite (pPnav).
When units of the Saint-Hélène complex form larger masses south of the arc and within the Sugluk Suture, they occur as competent masses. The first, close to Nallujuaq Lake, north of the Sugluk Suture, is felsic in its core and intermediate in the border. This kilometric mass has a strong magnetic signature and contains metasedimentary rock klippes (pPecv). Partial melting zones within tonalite are formed by pPshn4 diatexite. The latter is in transitional contact marked by an increase in the proportion of leucosome and therefore its heterogeneous appearance. However, paleosome or rafts of diorite and tonalite are preserved in the diatexite. Further south, in the Sugluk Suture, tonalite forms a rounded mass rimmed by quartz monzonite of the Suluraaq Suite (pPslq2).
Units of the Sainte-Hélène Complex are cut by speckled granodiorite of the Tasialuk Allipaaq Complex (pPali2a) and pegmatitic intrusions of the Sanningajualuk Suite (pPsnn1).
Does not apply.